Why do Muslims prostrate twice, but only bow once (in each Rak‘ah [unit of prayer])?

All perfect praise be to Allaah, the Lord of the worlds. I testify that there is none worthy of worship except Allaah and that Muhammad, sallallaahu ‘alayhi wa sallam, is His slave and Messenger.

Allaah Almighty imposed on His slaves to perform prayer in this form. Jibreel (Gabriel) taught the Prophet, sallallaahu ‘alayhi wa sallam, how to perform prayer and he, sallallaahu ‘alayhi wa sallam, said: "Pray as you have seen me pray." [Al-Bukhari]

Al-Buhooti, the Hanbali scholar explained the reason behind the repetition of prostration. He said:

"Prostration was legislated to be performed twice in each Rak‘ah, apart from all other actions in the prayer. This is because this position reflects the most humble state that a Muslim reaches after having stood up and bowed; he then achieves the highest level of worship by prostrating himself to his Lord. Hence, he is granted permission to sit (after rising from the first prostration) in servitude to his Lord. So he offers a second prostration out of gratefulness for the favor of being selected for servitude, being saved from the allurement of Satan, and being guided to the worship of the Most Merciful."

Allaah Knows best.